This invention relates to electromechanical actuators, and more particularly relates to actuator configurations for enabling efficient, large stroke actuation.
Electromechanical actuators are employed in a wide array of engineering systems, ranging from aerospace and automotive applications to microfabrication and printing applications. Generally, actuators are included in such applications to generate force and effect displacement, for example, to open or close valves, to deflect transmission linkages, to position components, or to enable another such system function. Discrete actuators employed for such functions typically are designed to provide a desired actuation stroke over which a desired force is delivered to a given load. In one class of discrete actuators, an active element, i.e., an element that is actively stimulated by, e.g., electrical, magnetic, or thermal stimulus, is provided to generate the desired force, and a support frame or other member configured with the active element is provided to translate relative motion of the active clement to a stroke having a related force for delivery to the load.
Based on this general discrete, active-element actuation mechanism, an actuator can be characterized by the level of energy it adds to or removes from the system in which it operates during the actuation stroke. Typically, energy must be supplied to the actuator from the system to enable the actuator stroke; specifically, energy is supplied to the active element to power generation of a force that is delivered through the stroke to a load, thereby producing work. A corresponding criterion for discrete actuator performance is generally based on efficiency of an actuator in converting energy input to its active element into the energy delivered by the actuator stroke. One metric for assessing this criterion is actuator mass efficiency, which is specified as the ratio of specific work delivered by an actuator to specific energy available to be supplied by the actuator's active elements.
Based on this energy relationship, the mass efficiency of a discrete actuator is directly related to the characteristic stiffness of the actuator, reflecting the fact that a stiff actuator load-bearing stroke mechanism is generally more efficient than a relatively more compliant stroke mechanism. Additionally, the actuator mass efficiency criterion is inversely related to the characteristic mass of an actuator, reflecting the fact that a relatively more massive stroke mechanism is generally more efficient than a less massive one. These mass efficiency considerations, both of which favor a more massive actuator, tend to be in direct conflict with the primary requirement of many engineering systems that the mass of an actuator incorporated in a such a system be minimized. But because engineering systems commonly require high actuator efficiency as well as low actuator weight, an explicit tradeoff in actuator design is typically required that often is suboptimal with respect to one or possibly even both requirements.
A wide variety of discrete, active-element actuator designs have been proposed in an effort to produce high-efficiency actuation mechanisms. For example, Hall and Spangler in U.S. Pat. No. 5,224,826, and Hall and Prechtl, in "Development of a piezoelectric servoflap for helicopter rotor control," Smart Mater. Struc., No. 5, pp. 26-34, 1996, have proposed an actuator, for helicopter rotor blade control, that employs an active monolithic piezoelectric ceramic bimorph structure cantilevered from a blade spar. The cantilevered structure is in turn connected to a trailing edge flap to be rotated by the actuator by way of a stroke mechanism incorporating three flexure points. While it is shown that tailoring of the bimorph structure can produce a relatively high actuator mass efficiency, the flexure points in the load-bearing actuation stroke path introduce parasitic compliance that inherently limits the attainable actuator mass efficiency, due to energy loss in bending, and for any actuator mass. In addition, the cantilevered monolithic bimorph structure, in which piezoelectric actuation is transverse to the applied electrical stimulus, is found to be characterized by an energy density that is considerably lower than that of a piezoelectric ceramic stack structure, in which piezoelectric actuation is parallel to the applied electrical stimulus.
While a piezoelectric ceramic stack structure indeed is characterized as an active actuation element having a high energy density, as well as a very high bandwidth, a piezoelectric stack structure is typically limited to only a relatively small stroke. As a result, a stroke amplification mechanism is generally required of an actuator incorporating such a structure. Ideally, an amplification stroke mechanism acts as nearly as possible like a true mechanism, i.e., it amplifies motion without resistance due to friction in hinges or other effects that impede mechanism motion, and it does not add compliance in series with the active element longitudinal expansion and contraction stroke path.
One discrete actuator employing an amplified-stroke ceramic stack configuration has been proposed by Kimura et al., in U.S. Pat. No. 5,447,381, among others. Here a piezoelectric stack is connected to a rigid support at one end and connected through a flexible member to a hinged fulcrum and lever structure at its other end. Expansion of the stack causes the fulcrum to rotate about the hinge away from the stack, in turn extending the lever, thereby translating the longitudinal stack expansion to a correspondingly amplified lever extension. Although this actuator provides a very effective stroke amplification mechanism, its lever design is inherently compliant, i.e., inefficient, in that it tends to bend against a load as the lever extends. Added mass is required to increase its stiffness and correspondingly increase its efficiency. Furthermore, realization of the hinge as a flexure that is located along the active element longitudinal expansion and contraction stroke path, as is conventional, introduces a high degree of parasitic compliance and energy loss that limits the actuator's efficiency for any selected mass.
The lever arm compliance of this fulcrum-lever design has been shown to be eliminated in a range of stack-based actuator designs that have been proposed, including, e.g., that of Stahlhuth in U.S. Pat. Nos. 4,808,874 and 4,952,835; and that of Fenn et al. in "Terfenol-D driven flaps for helicopter vibration reduction," SPIE V. 1917--Smart Struct. & Intell. Syst., pp. 407-418, 1993. In the Stahlhuth design, two active element stacks are positioned such that their longitudinal reaction creates an amplified perpendicular displacement of saggital linkages connected at ends of the stacks. In the Fenn design, two active element stacks are positioned to react longitudinally against each other at a shallow angle to create an amplified perpendicular displacement of a control rod connected between the stacks.
While overcoming the lever arm compliance of the lever design, both of these dual-stack actuator designs require at least one flexure to accommodate angular rotation of a stack or linkage relative to a support frame as the stacks longitudinally expand; indeed, several flexure points are required in each actuator. These flexures, being in the load-bearing stroke path, add significant parasitic compliance to the actuators and thus decrease the attainable efficiencies of the actuators for any selected actuator mass. The actuators are also rather complex and incompact in their configurations, thereby placing somewhat excessive space requirements on a system in which they are to operate. In addition, the Stahlhuth design requires compensation for thermally-induced excursions in its operation.
In another Stalhuth actuator design, similar to the Fenn actuator design described above and disclosed in U.S. Pat. No. 4,769,569, flexures at ends of the active element stacks are replaced by knife-edge rolling contact mechanisms. Although these contact mechanisms minimize flexural compliance, they are found to exhibit relatively high Hertzian losses due to their acute contact area. More importantly, this Stalhuth actuator employs a frame configuration in which the angle of the active element stacks, with respect to the horizontal, changes substantially over the course of one stroke cycle. As a result, the stroke amplification mechanism is very nonlinear, i.e., the stroke amplification factor changes during one stroke cycle. This nonlinearity is unacceptable for many high-precision applications, and in some cases can lead to a bifurcation of the active elements with respect to the frame.
These example actuator designs point out that actuator compliance is a predominant limitation conventionally associated with discrete actuator design inefficiency, due either to bending of a mechanism in the active element load path or to a hinge mechanism, such as a flexure, provided for accommodating rotational degrees of freedom in the active element load path. Compensation for bending with added actuator mass is often at odds with a given application weight limit, and cannot compensate for flexural compliance, which is parasitic and mass-independent. As a result, compromises in actuator performance specified for a given engineering application are often required to accommodate attainable actuator efficiency. Indeed, the example actuator designs illustrate that typically, an actuator configuration ultimately is limited to provide only one of low weight, high bandwidth, large stroke, or linearity performance advantages, but cannot well-address all four of these performance criteria will simultaneously meeting mass efficiency goals.